1. Field of the Invention
This invention relates to silicon nitride sintered bodies having an improved oxidation resistance and a method of producing the same and silicon nitride powders used for the production of silicon nitride sintered bodies.
2. Related Art Statement
Heretofore, a method of producing silicon nitride sintered bodies by adding a sintering aid, which forms a liquid phase at grain boundaries, such as Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO or the like, to commercially available silicon nitride starting powder, molding the resulting mixture into a given shape and then firing the shaped body in an inert gas atmosphere has hitherto been disclosed, for example, in Japanese Patent Application Publication No. 58-49509.
Such oxide additives, as well as SiO.sub.2 originally contained in the starting silicon nitride material and acting as a sintering aid, form glass in the intergranular phase during sintering and are effective for the densification and the formation of microstructure. However, the commercially available silicon nitride powder used as a starting material has generally an oxygen content of 1.2.about.2.0% by weight (2.2.about.3.8% by weight as converted into SiO.sub.2) and is less in the oxygen content and large in the fluctuation of oxygen content between lots, so that silicon nitride sintered bodies having generally a poor oxidation are frequently obtained.
Lately, there has been a strong demand to provide silicon nitride sintered bodies satisfying both oxidation resistance and high-temperature strength.